A. Field of the Invention
The present invention relates to a method and apparatus for exception processing in a pulse code modulation (xe2x80x9cPCMxe2x80x9d) communication environment. More particularly, the present invention relates to an in-band signal for exception processing in a PCM modern. A detection method and apparatus is also provided.
B. Description of the Related Art
For many years the public digital telephone network (xe2x80x9cDTNxe2x80x9d) has been used for data transmission between moderns. Typically, a modulated carrier is sent over a local loop to a service provider (e.g., a Regional Bell Operating Company), whereupon the service provider quantizes the signal for transmission through the DTN. A service provider that is located near the receiving location converts the digital signal back to an analog signal for transmidssion over a local loop to the receiving modem. This system is limited in the maximum achievable data rate at least in part by the sampling rate of the quantizers, which is typically 8 kHz (which rate is also the corresponding channel transmission rate, or clock rate, of the DTN).
Furthermore, the analog-to-digital (A/D) and digital-to-analog (D/A) conversions are typically performed in accordance with a non-linear quantizing rule. In North America, this conversion rule is known as xcexc-law. A similar nonlinear sampling technique known as A-law is used in other areas of the world such as Europe. The non-linear A/D and D/A conversion is generally performed by a codec (coder/decoder) device located at the interfaces between the DTN and local loops. Alternatively, these devices are referred to herein as a DAC (digital-to-analog converter) and an ADC (analog-to-digital converter).
It has been recognized that a data distribution system using the public telephone network can overcome certain aspects of t he aforesaid limitations by providing a digital data source connected directly to the DTN, without an intervening codec. In such a system, the telephone network routes digital signals from the data source to a client""s local subscriber loop without any intermediary analog facilities, such that the only analog portion of the link from the data source to the client is the client""s local loop (plus the associated analog electronics at both ends of the local loop). The only codec in the transmission path is the one at the DTN end of the client""s subscriber loop.
FIG. 1 shows a block diagram of a data distribution system. The system includes a data source 10, or server, having a direct digital connection 30 to a digital telephone network (DTN) 20. A client 40 is connected to the DTN 20 by an analog subscriber loop 50 that is typically a two-wire, or twisted-pair, cable. The DTN 20 routes digital signals from the data source 10 to the client""s local subscriber loop 50 without any intermediary analog facilities such that the only analog portion of the link from the server 10 to the client 40 is the subscriber loop 50. The analog portion of the system thus includes the channel characteristics of the subscriber loop 50 plus the associated analog electronics at both ends of the subscriber loop 50.
The analog electronics are well known to those skilled in the art and typically include a subscriber line interface card at the central office that includes a codec, as well as circuitry used to generate and interpret call progress signals (ring voltage, on-hook and off-hook detection, etc.). In the system of FIG. 1, the only codec in the transmission path from the server 10 to the client 40 is a DAC located at the DTN 20 end of the subscriber loop 50. It is understood that the client-side, or subscriber-side, equipment may incorporate an ADC and DAC for its internal signal processing, as is typical of present day modem devices. For the reverse channel, the only ADC converter in the path from the client 40 to the server 10 is also at the DTN 20 end of the subscriber loop 50.
In the system of FIG. 1, the server 10, having direct digital access to the DTN 20 may be a single computer, or may include a communications hub that provides digital access to a number of computers or processing units. Such a hub/server is disclosed in U.S. Pat. Nos. 5,528,595 and 5,577,105, the contents of which are incorporated herein by reference.
In the system shown in FIG. 1, digital data can be input to the DTN 20 as 8-bit bytes (octets) at the 8 kHz clock rate of the DTN. This is commonly referred to as a DS-0 signal format At the interface between the DTN 20 and the subscriber loop 50, the DTN 20 codec converts each byte to one of 255 analog voltage levels (two different octets each represent 0 volts, i.e plus 0 volts and minus 0 volts) that are sent over the subscriber loop 50 and received by a decoder at the client""s location. The last leg of this system, i.e., the local loop 50 from the network codec to the client 40, may be viewed as a type of baseband data transmission system because no carrier is being modulated in the transmission of the data. The baseband signal set contains the positive and negative voltage pulses output by the codec in response to the binary octets sent over the DTN. The client 40, as shown in FIG. 1, may be referred to herein as a PCM modem.
FIG. 2 is a graph showing a xcexc-law to linear conversion for one-half of the xcexc-law codeword set used by the DTN 20 codec. As shown in FIG. 2, the analog voltages (shown as decimal equivalents of linear codewords having 16 bits) corresponding to the quantization levels are non-uniformly spaced and follow a generally logarithmic curve. In other words, the increment in the analog voltage level produced from one codeword to the next is not linear, but depends on the mapping as shown in FIG. 2. Note that the vertical scale of FIG. 2 is calibrated in integers from 0 to 32,124. These numbers correspond to a linear 16-bit A/D converter. As is known to those of ordinary skill in the art, the sixteenth bit is a sign bit which provides integers from 0 to xe2x88x9232124 which correspond to octets from 0 to 127, not shown in FIG. 2. Thus FIG. 2 can be viewed as a conversion between the logarithmic binary data and the corresponding linear 16-bit binary data. It can also be seen in FIG. 2 that the logarithmic function of the standard conversion format is approximated by a series of 8 linear segments.
The conversion from octet to analog voltage (or a digital representation of the analog voltage, as discussed above) is well known, and as stated above, is based on a system called xcexc-law coding in North America and A-law coding in Europe. Theoretically, there are 256 points represented by the 256 possible octets, or xcexc-law codewords. The format of a xcexc-law codeword is shown in FIG. 3, where the most significant bit b7 indicates the sign of the codeword, and the remaining seven bits, b6-b0, represent the magnitude of the codeword. Referring to FIG. 2, it may be observed that the three bits b6-b4 represent the linear segment in the conversion graph, and the four bits, b0-b3 indicate the step along the particular, linear segment in the conversion graph. These points are symmetric about zero; i.e., there are 128 positive and 128 negative levels, including two encodings of zero. Since there are 254 non-zero points, the maximum number of bits that can be sent per signaling interval (symbol) is just under 8 bits.
A xcexc-law or A-law codeword utilized by the DTN 20 may be referred to herein as a PCM codeword. It is actually the PCM codeword that results in the DTN 20 codec to 5 output a particular analog voltage. The codeword and the corresponding voltage may be referred to herein as xe2x80x9cpoints.xe2x80x9d The client 40, shown in FIG. 1, may be referred to herein as a PCM modem.
Other factors, such as robbed-bit signaling, digital attenuation (pads), channel distortion and noise introduced by the subscriber loop, and the crowding of points at the smaller voltage amplitudes and the associated difficulty in distinguishing between them at the decoder/receiver, may reduce the maximum attainable bit rate. Robbed Bit Signaling (RBS) involves the periodic use of the least significant bit (LSB) of the PCM codeword by the DTN 20 to convey control information. Usually the robbed bit is replaced with a M4 logical xe2x80x981xe2x80x99 before transmission to the client 40. Typically, the DTN performs robbed-bit signalling on a cyclic basis, robbing the LSB of an individual channel every sixth PCM codeword. In addition, due to the fact that a channel might traverse several digital networks before arriving at the terminus of the DTN 20, more than one PCM codeword per 6 time slot frame may have a bit robbed by each network, with each network link robbing a LSB.
During a communication session, the client device, whether a PCM modem or a conventional modem such as a V.34 modem, typically proceeds to steady state operation after initial start-up and training procedures are executed. As used herein, the term V.34 modem means a modem that complies with Recommendation V.34 (1994) as established by the International Telecommunication Union, Telecommunication Standardization Sector, the contents of which are incorporated herein by reference. Steady state operation refers to the reception and transmission of data, as opposed to control signals and the like, by the client device. Steady state operation is sometimes referred to as the xe2x80x9cdata mode.xe2x80x9d Over the course of the communication session, it may be desirable to change the mode of the client device, such as switching the client device out of steady state operation. Exception processing, as the term is used herein, refers to techniques and/or associated hardware/software for indicating the desirability of changing the mode of the client device. A mode change for the client device may also be referred to herein as an exception. Typical exceptions for conventional modems include retrains and rate renegotiation.
There are known exception processing techniques. For example, while a V.34 modem is in session it is often the case that changing conditions of the transmission medium require that a different modulation rate be used for an optimal (in terms of the speed v. reliability tradeoff) rate of data exchange. In accordance with Recommendation V.34, the request for rate renegotiation is handled by generating a unique signal sequence that is not within the set of signalling points used for the transmission of data. Exception to processing techniques that use unique signal points, which are not within the set of signalling points used for the transmission of data, may be referred to herein as xe2x80x9cout-of-bandxe2x80x9d techniques.
FIG. 4 illustrates a four point constellation that is used by V.34 modems to request a rate renegotiation. In FIG. 4, the 0""s represent the points that are sent to request a rate renegotiation, whereas the x""s represent QAM constellation points for V.34. Because the points for requesting a rate renegotiation (e.g., 0""s) are not within the set of signalling points used for the transmission of data (e.g., x""s), this is an example of an out-of-band signal for exception processing. A V.34 modem will typically have an independent detection mechanism for detecting the rate renegotiation request.
A disadvantage of this approach is that the independent detection mechanism unduly complicates modem hardware and/or consumes processing resources, instruction time or code space if the independent detection mechanism is implemented algorithmically within a digital signal processor. A further disadvantage of this approach arises in the context of the PCM modem. Specifically, the out-of-band signal may lead to divergence of any adaptive mechanisms that use equalizer error for convergence in the PCM modem.
It would therefore be desirable to have an improved method and apparatus for exception processing.
In accordance with a first aspect of the present invention, a signal for exception processing is provided. The signal includes a first sequence of PCM codewords. Each PCM codeword in the sequence has a magnitude corresponding to a maximum point within an active constellation. The signal further includes a timing mark following the first sequence.
In accordance with a second aspect of the present invention, a client device is provided. The client device includes a receiver and a detector coupled to the receiver. The receiver is coupled to an analog subscriber loop and has a decision feedback equalizer. The receiver provides a decision corresponding to data sent by a digital data source. The detector monitors the decision of the receiver to determine when an exception has been requested by the digital data source.
In accordance with a third aspect of the present invention, a method of requesting a change in a mode of operation of a client device when the client device is operating in a data mode is provided. The data mode includes receiving signal points selected from an active constellation and deciding which points have been received to thereby recover data. The method includes the step of transmitting, to the client device, a predetermined sequence of points from the active constellation. Each of the points in the predetermined sequence has the same index relative to the maximum point within the active constellation. The method further includes the steps of receiving the transmitted sequence, and detecting the predetermined sequence.